When I run "python main.py fullobs_collect_treasure V1" I meet error "ImportError: cannot import name 'Wall'"

[leeruibin]

The error log is as follows:
File "", line 678, in exec_module
File "", line 219, in _call_with_frames_removed
File "D:\project_code\python\yiwei\MAAC-master\envs\mpe_scenarios\fullobs_collect_treasure.py", line 3, in
from multiagent.core import World, Agent, Landmark, Wall
ImportError: cannot import name 'Wall'

It seems that the there is no "Wall" class in the multiagent repo, i search the the core.py file in multiagent repo https://github.com/openai/multiagent-particle-envs and find World, Agent, Landmark. But there is no Wall class. Do you change your multiagent.core file? what is the defination of the Wall class? Thanks.